Accelerometer manufacturing method

ABSTRACT

A method for manufacturing a compound accelerometer that is maximally sensitive to a desired acceleration vector to the exclusion of orthogonal vectors. The compound accelerometer is made by combining two simple accelerometer units. The simple accelerometer units consist of a hollow container closed at each end by a piezo-electric transducer. The two transducers are oppositely polarized and are selected to have a desired combined series capacitance value. The container is partially filled with a volume of heavy liquid and the accelerometer is excited. During excitation, the output voltage is continuously measured and the volume of liquid is microadjusted until the output voltage equals a prescribed level. The outputs of the individual transducers are measured, the residual voltage difference is tabulated and the dominant transducer is identified. Two simple accelerometer units having the same residual voltage difference are secured together with the dominant transducers facing oppositely with respect to each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to accelerometers having a liquid inertia mass.

2. Description of the Prior Art

Accelerometers used in seismic exploration may take the form of acylindrical chamber, closed at each end by a piezo-electric wafer. Aquantity of mercury fills most of the volume of the closed chamber andserves as a liquid inertia mass. In most orientations of the principalaxis of such an accelerometer unit, the accelerometer output signal isaffected by both vertical and horizontal accelerating forces. One or theother of the acceleration vectors can be isolated by additively orsubtractively combining the respective electrical outputs of the twopiezo-electric wafers. One such accelerometer is disclosed in U.S. Pat.No. 4,334,296, issued June 8, 1982 to E. M. Hall, Jr., which isincorporated herein by reference.

The degree of cancellation of an unwanted acceleration vector depends ofcourse, on the precision with which the two wafers can be matched as totheir electrical and mechanical parameters. It has been our experiencethat it is impractical from a commercial production standpoint tohand-pick unmounted wafers that match exactly. We have also found thatwafers which match precisely before assembly, commonly become mismatchedduring the assembly process.

One method for matching the sensitivities of the two piezo-electricwafers after assembly is disclosed in U.S. Pat. No. 4,395,908 issuedAug. 8, 1983 to R. C. Shopland. Another method is disclosed in aco-pending U.S. patent application Ser. No. 517,899, now U.S. Pat. No.4,455,500 in the name of C. H. Savit et al. Although those methodsserved their intended purpose of matching the mechanical and electricalparameters of the wafers of an assembled accelerometer, the methods tendto complicate the commercial manufacturing operation.

SUMMARY OF THE INVENTION

It is a purpose of this invention to provide a method for manufacturinga compound accelerometer that will be maximally sensitive to a desiredacceleration vector to the virtually complete exclusion of any residualsignal due to an unwanted orthogonal acceleration vector, and one thatwill be practical for commercial production.

In accordance with a preferred embodiment of our invention, we selectseveral sets of two oppositely-polarized piezo-electric transducers fromamong a larger number of such transducers. Each set of transducers ischaracterized by a predetermined combined capacitance value and amatched set of output voltage sensitivities per unit force input equalto at least a threshold value. The two transducers of a set are mounted,in spaced-apart relationship, in a container. The container is partiallyfilled with a prescribed volume of a heavy liquid thereby to provide asimple accelerometer unit. The accelerometer unit is excited by adesired accelerating force at a specified frequency. The total voltageoutput of the accelerometer unit is continuously measured while underexcitation and the volume of liquid is micro-adjusted until the outputvoltage equals a prescribed value. The output voltage of each individualtransducer is then measured to determine a residual voltage and thedominant transducer of the two is identified. A plurality of simpleaccelerometer units are sorted by pairs such that the two units of apair are exactly matched as to residual voltage. They may be furthermatched as to total capacitance. The two simple accelerometer units aresecured together with the dominant transducers facing oppositely withrespect to each other. The resulting compound accelerometer isinsensitive to acceleration in the undesired direction to the extent ofthe precision with which the matching of the simple accelerometers isachieved.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the benefits and advantages of our inventionmay be obtained from the appended detailed description and the drawings,wherein:

FIG. 1 is a cross section of a simple accelerometer unit having a liquidinertia mass;

FIG. 2 is an end view of the accelerometer unit of FIG. 1; and

FIG. 3 is a cross-sectional view of the compound accelerometer of thisinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, there is shown a cross section of a simpleaccelerometer unit 10 whose principal components will be describedbriefly. The unit consists of a conductive cylindrical case or chamber12 having inner and outer shoulders 14, 16 and 14', 16' at each end.Thin outer walls 18, 18' that extend beyond outer shoulders 16, 16'later may be crimped over as shown.

The ends of chamber 12 are closed by transducers 20, 20'. Thetransducers, of which 20 is typical, are composed of a thin metaldiaphragm or substrate 22 to which is cemented a ceramic piezo-electricwafer 24 using a conductive epoxy resin. The left hand exposed face ofwafer 24, for example, in FIG. 1 constitutes the free-air face of thattransducer.

Before assembly, the piezo-electric wafers are polarized. By seismicindustry standards a wafer is said to be positively polarized if, when acompressive force is applied against the free-air face, the free-airface develops a positive charge relative to the substrate. In theassembly of FIG. 1, the two wafers are oppositely polarized.

Simple accelerometer unit 10 is assembled by seating transducers 20, 20'on shoulders 14, 14' respectively, followed by O-rings 28, 28'.Retaining rings 30, 30', preferably brass, are seated on outer shoulders16, 16' and are held in place by crimping thin walls 18, 18' over theretaining rings. At this point, the assembly operation is not criticalexcept that the retaining rings must not touch the substrate and must beevenly secured in place. The free air faces of the piezo-electric wafersare metal-plated so that leads 26, 26' may be soldered thereto. Ifdesired, the free air faces of the piezo-electric wafers may be coatedwith a sealant 32 such as but not limited to a polysulfide resin. A fillhole 33 and a cap screw 34 are provided for filling the chamber 12 witha volume of heavy liquid having a specific gravity of at least 9.6 suchas mercury, 36, as will be later discussed. An electricalseries-connection between the two transducers is established through theconductive case 12.

FIG. 2 is an end view of the accelerometer unit of FIG. 1 wherein thesame reference numbers represent the same parts in the two figures.

In general it is contemplated that in normal use, the accelerometer unitwill be operated in the orientation shown in FIG. 1. That is, an axialline parallel to the principal structural axis which passesperpendicularly through the transducers will be approximately orthogonalto the gravitational vertical. The orientation is not critical however.

In the '296 reference patent, the two oppositely-mounted transducerpiezo-electric wafers have the same polarity. The two leads of eachtransducer may be brought out separately so that the transducer outputvoltages could be combined externally, either additvely orsubtractively. In our embodiment, we prefer to minimize the internalwiring by using the conductive case to complete a series circuit betweentwo oppositely-polarized transducers.

In FIG. 1, the spacing between the transducers is shown to be relativelysmall with respect to their diameter. From well-known physicalprinciples, for a given transducer spacing, the output voltagesensitivity of the transducers is inherently a function of the length ofthe mercury column between the transducers as measured along a linedirected parallel to the desired acceleration vector. The electrical andmechanical design as shown in FIG. 1, by way of example but not by wayof limitation, tends to favor a vertical acceleration vector.

So far, we have discussed the general method of construction of anaccelerometer unit having a liquid inertia mass contained within acylindrical chamber that is closed at each end by a transducer element.We shall now list in detail, the manufacturing steps required to providea compound accelerometer that is maximally sensitive to a desiredacceleration vector and which is capable of substantially cancelling theoutput signal due to an unwanted acceleration vector.

A quantity each of oppositively-polarized transducers are prepared. Thecapacitance of each transducer is measured to three significant places.The transducers are then marked and sorted by their capacitance, with anindividual bin for each incremental value. Typically, the capacitance,for an exemplary lot of transducers, might range from 12.0 to 16.5nanofarads (nf). Transducers whose capacitance falls outside that rangeshould be discarded. There are now two groups of sorted transducers: onegroup being positively polarized and one group being negativelypolarized.

In the same operation that the capacitance is measured, each transduceris mounted on a fixture on a shaking table where it is driven at aspecified acceleration level at a desired frequency under prescribedloading conditions. That is, a floating solid inertia mass is applied tothe center of the transducer by the test fixture. For example theshaking table might drive the transducers at 1 in/sec² at 70 Hz. Thethreshold output voltage of the transducers should be, for example, atleast 30 millivolts (mv). Transducers having a lesser output should bediscarded. The value of the measured output voltage is marked on eachtransducer with ink or other permanent marking means.

Next, pairs of oppositely-polarized transducers are selected such thatthe two selected transducers have a combined series capacitance of 7 nf±5% and threshold output voltages that match within about ±3%. Each pairof transducers is then assembled into a complete simple accelerationunit as previously described. The completed unit is assigned a serialnumber for identification.

The completed units are then nearly filled with a precisely specifiedquantity of a heavy liquid, preferably mercury. The nearly-filledaccelerometer unit (about 90% full) is placed on the shaking table andis excited at a desired acceleration level at a desired frequency. In anexemplary test, the quantity of mercury was 1.4 cc and the drive levelwas 10 in/sec² at 70 Hz. While the accelerometer unit is being driven,the output voltage is continuously measured. Manually, by means of ametering syringe or automatically by a pumping system, the output of theaccelerometer unit is micro-adjusted by adding or removing micro-volumesof mercury from the accelerometer unit until the output voltage reachesa specified value ±1 mv. In the exemplary test, the specified value was20 mv. Thereafter the cap screw is replaced. The accelerometer output isagain measured to insure that insertion of the cap screw has notdisturbed the calibration.

Finally, the output voltage of each individual transducer, measuredrelative to the conductive case 12, is measured and tabulated on alisting as a function of the unit serial number. Thus, there will be apositive voltage and a negative voltage for each unit. A residualvoltage, let us call it X, is determined by subtracting the absolutevalue of the negative voltage from the positive voltage. The sign of Xdetermines whether the positively or the negatively polarized transduceris electrically dominant. That is, the dominant transducer is thetransducer that is characterized by the greater voltage sensitivity. Thesigned value of X and the total capacitance of the accelerometer unitare entered on the listing. The listing is computer-sorted first byresidual voltage increments and then by increments in capacitance. Pairsof simple accelerometer units, identified by serial number, that areexactly matched as to the numerical value of the residual voltages areselected from the listings. A mismatch in total capacitance is of lessimportance although it is preferable to pair-off a simple accelerometerunit having a higher total capacitance than average with another unithaving a lower-than-average capacitance, provided only that residualvoltages X are equal.

Referring now to FIG. 3, a plastic case 40 is provided. A pair ofmatched simple accelerometer units 42, 44 are inserted into the case,separated by an O-ring 46 and are secured together. It is essential thatthe electrically dominant transducers such as 48, 50 of accelerometers42 and 44 face in opposite directions. Whether the electrically dominanttransducers face each other or face away from each other as shown isunimportant; it is important that they face oppositely. The twoaccelerometers may be electrically connected in parallel or in series.If the compound accelerometer is to work into a charge-coupledpreamplifier, the simple accelerometer units should be connected inparallel to obtain the highest possible amplifier sensitivity.Otherwise, such as for transformer coupling, they may beseries-connected. Once secured together in place, the simpleaccelerometer units are sealed in by an end cap 52 having an exit portfor accelerometer leads 54. The design details of case 40 areunimportant and are commensurate with the service to which the compoundaccelerometer is to be subjected, i.e. dry land or deep marineenvironments.

For illustrative purposes, our invention has been described with acertain degree of specificity. Variations in the method of manufacturewill occur to those skilled in the art but which lie within the scope ofthe inventive process which is limited only by the appended claims.

We claim as our invention:
 1. A method for manufacturing a compoundaccelerometer that is maximally sensitive to a desired accelerationvector to the substantially complete exclusion of unwanted accelerationvectors, comprising the steps of:selecting several sets of two,oppositely-polarized piezo-electric transducers, from among a pluralityof transducers, each said set being characterized by a predefinedcombined series capacitance value and an output voltage sensitivity atleast equal to a desired threshold value; mounting, in spaced-apartrelationships, each said set of two transducers in individual containersthereby to provide several simple accelerometer units; inserting apredetermined volume of a liquid inertia mass into each said container;exciting said simple accelerometer units at a prescribed accelerationlevel and a prescribed frequency; continuously measuring the totaloutput voltages of each of the several simple accelerometer units andmicro-adjusting the volumes of the liquid masses during excitation untilthe total output voltage of each unit equals a predetermined value;measuring and tabulating the residual output voltage differences betweenthe individual transducers of each said simple accelerometer unit andidentifying the electrically dominant transducer of each unit; sortingthe simple accelerometer units by pairs such that the two simpleaccelerometer units of each pair are substantially exactly matched as toresidual output voltages; and securing together the two matched units ofeach pair with the electrically dominant transducers facing in oppositedirections with respect to one another.
 2. The acceleration sensormanufacturing method as defined by claim 1, including the stepof:matching a simple accelerometer unit having a higher-than-averagetotal capacitance with another simple accelerometer unit having alower-than-average total capacitance, both having the same residualvoltage.
 3. The acceleration sensor manufacturing method as defined byclaim 1, wherein the step of selecting includes the step of:excitingeach said piezo-electric transducer at a desired acceleration level, ata prescribed frequency, and under pre-specified conditions of transducerloading, to determine the output voltage sensitivity relative to adesired threshold value.
 4. The acceleration sensor manufacturing methodas defined by claim 3 wherein the step of selecting furtherincludes:measuring the capacitance of each one of said transducers andsorting said transducers by capacitance increment and by polarization.5. The acceleration sensor manufacturing method as defined by claim 1wherein said step of exciting includes the step of:emplacing said simpleaccelerometer units on a shaking table with their principal structuralaxes oriented horizontally.